This invention relates to power modules and more particularly to a heatsink retainer for fastening a heatsink to a power module.
Power modules all include power electronic devices such as MOSFETs, power diodes, IGBTs, power bipolar transistors, etc. These devices generate heat during their operation which adversely affects their performance. Therefore, in every module design extraction and dissipation of generated heat is an important design concern.
In many power modules, the power electronic devices are mounted on a thermally conductive substrate such as an insulated metal substrate (IMS). The devices are thermally coupled to the substrate so that their generated heat may be transferred to the substrate during operation. The substrate is then placed in intimate contact with a heatsink. The heatsink through its intimate contact with the substrate draws heat that is transferred from the power electronic devices to the substrate away from the substrate and dissipates it to the ambient. Thereby, heat is extracted from the power electronic devices and dissipated and thus the power electronic devices are kept relatively cool during operation.
Typically, a heatsink is an extruded plate of thermally conductive material such as a thermally conductive metal, for example, aluminum. Some heatsinks are extruded with fins that are integral with and extend away from one surface of the heatsink base, i.e. the portion of the heatsink that makes contact with the substrate. The fins provide for additional surface area which improves the cooling performance of the heatsink. In some heatsink designs narrow slots are cut through the fins in order to increase the area on the edges of the fins to provide for air flow across the fins, which also increases cooling performance of the heatsink. The slots in these heatsinks may be deep enough to reach the base of the heatsink.
Many conventional methods are used to place a heatsink in intimate contact with a heat producing electronic device. For example, in the construction of Pentium(copyright) chip cooler assemblies, it is standard practice to fasten an extruded heatsink to the Pentium(copyright) chip using a light force spring metal stamping or a clip. The clip is designed to pass between extruded fins to contact and pressure the bottom surface of the heatsink base to the Pentium(copyright) chip. Such clips include arms which extend roughly perpendicular to the heatsink base and engage corresponding hooks on the plastic casing of the Pentium(copyright) chip. In some cases, the mounting hardware (e.g. clip) is perpendicular to (across) the direction of the extrusion of the fins. Where it is necessary to mount the clip perpendicular to the fins of the heatsink, it is common practice to remove fin material from all fins to create the necessary clearance for the clip. Thus, a typical clip used to place a heatsink to a Pentium(copyright) chip consumes space on either side of the extrusion which may be needed for other components.
In many power module designs, the substrate and the heatsink are disposed below a power circuit board (PCB). The PCB typically has disposed thereon electronic control elements which control the power electronic devices. The electronic control elements are interconnected via, for example, copper traces that lay atop the PCB. In some modules, an opening is provided in the PCB, and the control elements and the power electronic devices are connected together via wire bonds that reach the power electronic devices through the opening. For example, in some designs the copper traces terminate at bond wire posts which are provided atop the PCB and disposed at or near the opening in the PCB. The bond wires are then connected to the bond wire posts at one end and connected to an electrical contact on a respective power electronic device on the substrate at the other end. Other means, for example vias, can be used to electrically connect the control elements on the PCB and the power electronic devices without providing an opening in the PCB.
To ensure that the heat is properly transferred, the substrate and the heatsink are held closely together. Where the substrate and the heatsink are disposed below the PCB, the heatsink is assembled on the PCB with fasteners. To accommodate the fasteners, the PCB typically includes mounting holes which extend through the body of the PCB. These holes take away space on the PCB. Moreover, since the fasteners are typically grounded, the copper traces on the PCB must be placed far from the site of the fasteners due to creepage and strike concerns. This is a drawback in module design. This drawback is particularly amplified in modules that employ a PCB that has an opening therein for bond wires in that areas around the opening have considerably less surface area. Therefore where the fasteners are located near the opening (as they typically are) the proper positioning of copper traces on the PCB is difficult.
It is therefore desirable to have a module that does not suffer from the drawbacks of these conventional arrangements.
A power module according to the present invention includes a heatsink retainer for assembling a heatsink onto the module. A heatsink retainer according to the present invention mounts a heatsink without the necessity of providing mounting holes or mounting notches on the PCB which results in conserving board space that can be used for current carrying copper traces.
A power module having a retainer according to the present invention includes a power shell which has embedded therein a thermally conductive power substrate such as an IMS. The power shell includes snaps on two opposing edges thereof. The snaps fit onto corresponding protrusions on a PCB so as to mount the power shell to the underside of the PCB. In a preferred embodiment, the power shell includes at least one flange that fits into a corresponding slot in the body of the PCB to prevent the power shell from moving along the edges of the PCB.
The power shell also includes two heatsink retainer posts which extend from the underside of the power shell. The heatsink retainer is mounted onto the underside of the power shell by the heatsink retainer posts to bias a heatsink to the thermally conductive substrate.
A heatsink retainer according to the present invention includes two blade portions each having a free edge for making contact with a heatsink, and two U-shaped brackets each being connected to an end of one of the blades. According to another aspect of the invention, the edges of the heatsink retainer that make contact with the heatsink are arcuate.
According to one embodiment of the invention, the power shell includes two heats ink retainer posts, each being disposed on an opposing edge of the thermally conductive substrate. One of the retainer posts is U-shaped, and the other is bar-shaped. The U-Shaped retainer post provides an opening that receives one of the brackets of the heatsink retainer while the other bracket of the heatsink retainer fits over the top of the bar-shaped retainer post. In a preferred embodiment, the bar-shaped retainer post includes a blind mounting hole for receiving a mounting fastener that is placed through the bracket on the heatsink retainer. Preferably, one or more snaps may be provided on either side of the bar-shaped retainer post to snap into corresponding openings on the bracket that is placed over the bar-shaped retainer post.
According to another embodiment, there are two heatsink retainer posts that extend from the underside of the power shell and are disposed on the opposing edges of the thermally conductive substrate. Each one of the heat sink retainer posts in this embodiment receives a bracket on the heatsink retainer in an identical manner.
According to an aspect of the present invention, the heatsink, includes a plurality of laterally positioned fins extending from a heatsink base, and a plurality of laterally spaced slots each extending orthogonal to the direction of elongation of the fins. The blades of the heat sink retainer fit inside of at least two laterally spaced slots and are placed in contact with the heatsink base.
According to another aspect of the present invention the blades of heatsink retainer may include several openings so as to not interfere with the passage of cooling air between the fins of the heatsink.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.